Author here. Not all HDMI/HDCP implementations work like this. Basically, the designer of the SoC (NXP) will have licenced an HDMI/HDCP IP block from some company selling such IP. It looks like NXP licenced their HDMI/HDCP IP from Cadence in this case, a major provider of silicon IP.

So this particular HDMI/HDCP implementation is designed to require this signed blob in order for it to function. Could Cadence have designed it so you can use it without HDCP without this blob, or use an unsigned blob and forego HDCP support, probably. But they didn't, so that's the situation.